Cross Bridge Activation

Human Anatomy And Physiology Premium Course

Human Anatomy and Physiology Study Course

Get Instant Access

The thin filaments in smooth muscle do not have the calcium-binding protein troponin that mediates calcium-triggered cross-bridge activity in both skeletal and cardiac muscle. Instead, cross-bridge cycling in

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Smooth Muscle Electron Micrograph

FIGURE 11-36

Electron micrograph of portions of three smooth-muscle fibers. Higher magnification of thick filaments (insert) with arrows indicating cross bridges connecting to adjacent thin filaments.

From A. P. Somlyo, C. E. Devine, Avril V. Somlyo, and R. V. Rice, Phil. Trans. R. Soc. Lond. B, 265:223-229, (1973).

FIGURE 11-36

Electron micrograph of portions of three smooth-muscle fibers. Higher magnification of thick filaments (insert) with arrows indicating cross bridges connecting to adjacent thin filaments.

From A. P. Somlyo, C. E. Devine, Avril V. Somlyo, and R. V. Rice, Phil. Trans. R. Soc. Lond. B, 265:223-229, (1973).

smooth muscle is controlled by a calcium-regulated enzyme that phosphorylates myosin. Only the phospho-rylated form of smooth-muscle myosin is able to bind to actin and undergo cross-bridge cycling.

The following sequence of events occurs after a rise in cytosolic calcium in a smooth-muscle fiber (Figure 11-37): (1) Calcium binds to calmodulin, a calcium-binding protein that is present in most cells (Chapter 7) and whose structure is related to that of troponin. (2) The calcium-calmodulin complex binds to a protein kinase, myosin light-chain kinase, thereby activating the enzyme. (3) The active protein kinase then uses ATP to phosphorylate myosin light chains in the globular head of myosin. (4) The phosphorylated cross bridge binds to actin. Hence, cross-bridge activity in smooth muscle is turned on by calcium-mediated changes in the thick filaments, whereas in striated muscle, calcium mediates changes in the thin filaments.

The smooth-muscle myosin isozyme has a very low maximal rate of ATPase activity, on the order of 10 to 100 times less than that of skeletal-muscle myosin. Since the rate of ATP splitting determines the rate of cross-bridge cycling and thus shortening velocity, smooth-muscle shortening is much slower than that of skeletal muscle. Moreover, smooth muscle does not undergo fatigue during prolonged periods of activity.

To relax a contracted smooth muscle, myosin must be dephosphorylated because dephosphorylated myosin is unable to bind to actin. This dephosphory-lation is mediated by the enzyme myosin light-chain phosphatase, which is continuously active in smooth muscle during periods of rest and contraction. When cytosolic calcium rises, the rate of myosin phosphorylation by the activated kinase exceeds the rate of de-phosphorylation by the phosphatase, and the amount of phosphorylated myosin in the cell increases, producing a rise in tension. When the cytosolic calcium concentration decreases, the rate of dephosphoryla-tion exceeds the rate of phosphorylation, and the amount of phosphorylated myosin decreases, producing relaxation.

If the cytosolic calcium concentration remains elevated, the rate of ATP splitting by the cross bridges declines even though isometric tension is maintained. When a phosphorylated cross bridge is dephosphor-ylated while still attached to actin, it can maintain

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Muscle CHAPTER ELEVEN

Muscle CHAPTER ELEVEN

Smooth muscle Skeletal muscle

Smooth muscle Skeletal muscle

Smooth Muscle Calcium Calmodulin

FIGURE 11-37

Pathways leading from increased cytosolic calcium to cross-bridge cycling in smooth- and skeletal-muscle fibers.

FIGURE 11-37

Pathways leading from increased cytosolic calcium to cross-bridge cycling in smooth- and skeletal-muscle fibers.

tension in a rigorlike state without movement. Dissociation of these dephosphorylated cross bridges from actin by the binding of ATP occurs at a much slower rate than dissociation of phosphorylated bridges. The net result is the ability to maintain tension for long periods of time with a very low rate of ATP consumption.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • Karolin
    What sequence of events occurs to form a cross bridge?
    8 years ago
  • Torsten Pfeifer
    How does calmodulin binding calcium result in crossbridge attachment?
    7 years ago

Post a comment